The invention herein relates to hollow glassy spheres (sometimes referred to as "microspheres" or "microballoons") and the production thereof.
In the past generally spherical inorganic hollow particles have been produced in a variety of different manners and from several different types of materials. U.S. Pat. No. 2,676,892 discloses a process for the formation of hollow spheres from high melting point naturally occurring argillaceous materials such as clay. These materials are first dried, removing free water and, if desired, also the small (about 4-5 weight percent) of combined water. The dried materials are then powdered and then suspended in a hot gaseous medium at temperatures of about 1100.degree. to 2200.degree. C., usually 1400.degree. to 2200.degree. C., to expand the argillaceous particles. Because of the high fusion temperature of the materials and the scarcity or complete lack of water therein, the process described in the patent relies entirely on the reduction of ferric oxide in the materials to yield oxygen as a gaseous expanding agent. In order for the process to work the ferric oxide content must be at least 2 weight percent, preferably about 6 weight percent. It is reported (in subsequent U.S. Pat. No. 3,030,215) that the spherical particles produced in this process are "of poor uniformity, of relatively high density, and . . . of an uneven and irregular surface . . . ".
The aforesaid U.S. Pat. No. 3,030,215 describes a process for forming spheres from a mixture of an alkali metal silicate, an oxide silicate insolubilizing agent and a solid blowing agent. Considerable preparation of the feed material, including slurrying and drying of the silicate, is required. A somewhat similar process is shown in the U.S. Pat. No. 3,794,503, which involves the formation of spheres by spray drying of a mixed solution of an alkali metal silicate and an organic polysalt.
U.S. Pat. No. 3,365,315 describes a two-step process for forming hollow glass spheres from conventional glass forming mixtures of oxides. The oxides are first melted to form a glass and then cooled and solidified. They are then subjected to a high humidity or gaseous atmosphere to adsorb or absorb sufficient gas forming material to be expanded in a subsequent reheating step. A solid gas forming material may as an alternative be incorporated into the initial glass, which will in the subsequent reheating step liberate a gaseous blowing agent. While the glass spheres so produced are relatively uniform and of good quality, the addition of a liquid or gaseous material to a glass melt is a very difficult operation and hazardous when high temperatures are involved. The necessity to utilize two distinct heating steps separated by a cooling step also represents a significant degree of complexity.
U.S. Pat. Nos. 2,797,201 and 3,129,086 disclose the expansion of spheres from a film forming material by use of a latent gas material dissolved in a suitable solvent.
It will be recognized that these prior art processes involve the use of complex or hazardous production methods, require very specialized raw materials or produce unsatisfactory end products. It would therefore be advantageous to have a process for the formation of hollow glassy spheres of good quality which is simple to perform, utilizes readily available raw materials, does not involve exotic or hazardous operating methods and produces a satisfactory end product in good yield.
It has also been known in the past that perlite, a rhyolitic volcanic glass, can be expanded into irregular shapes by rapid heating. Powdered perlite is passed rapidly through a high temperature zone and the heat causes the combined water in the perlite to convert rapidly to steam, causing the perlite to "pop" and form hollow irregular particles. A typical process is described in U.S. Pat. No. 2,639,132. Perlite expansion, however, is based on different mechanism from the zeolite expansion of this invention. Perlite, which is vitreous rather than crystalline, expands by softening rather than melting, and therefore expands over a range of temperatures, which results in the irregularities and fractions in the expanded perlite product. Perlite also has a highly variable water content, which leads to irregularities and fractures in the product.